In the fabrication of large diameter pipes, steel workpiece sheets are rolled and formed into a cylindrical shape, and lateral edges of the rolled sheet are joined to form a longitudinal pipe seam. The seam joint is typically welded to finish the cylinder shape, where the welded seam must be of adequate strength to maintain fluid seal for transfer of fluids or gas within the finished pipe. Prior to engaging the pipe seam edges, the edges are normally ground or milled to create angled or chamfered portions of the edges, where engagement of the seam edges forms inner and outer longitudinal grooves along the seam. In welding the pipe seam, it is initially desirable to tack weld the seam edges to one another to create a joint for holding the seam together. Thereafter, inner and outer welding steps fill the remainder of the seam grooves, wherein the initial tack weld is completely consumed by the subsequent inner and outer groove welds. The initial tack weld ideally can have no holes and must be structurally sound to hold the pipe together until the subsequent welds are completed. In creating the initial tack weld, the welder is preferably translated along the pipe seam at a high speed to provide sufficient molten metal to hold the seam edges together with no gaps and without damaging the edges of the pipe seam.
Pipe milling operations are demanding high lineal welding speeds for the initial seam tack weld, which welding speeds require higher weld deposition rates than can be produced with current seam welding techniques. In particular, pipe fabricators want to be able to tack weld pipe seams at lineal weld speeds as high as 10 meters per minute, whereas conventional pipe tack welding operations can achieve weld rates of only about 3 to 5 meters per minute. In conventional pipe welding, the milled seam edges are initially brought in contact and a tack weld is performed at a voltage of about 25 volts with 600 to 800 amperes current using short-circuit transfer techniques with relatively thick welding wire (e.g., ⅛ or 5/32 inch wire). This is normally done by DC or AC MIG welding to provide high welding speed. However, such high deposition rate processes involve high arc force, in which the arc may tend to drive-through the pipe. Sometimes short circuit arc processes are used for tacking the seam, with process controls adapted to control the arc force and workpiece heating, at the expense of welding rate. Such a process employs a short-circuit arc transfer welding procedure using a wire feed speed that is normally in the spray range, with a voltage well below a normal spray arc voltage so as to cause the short-circuit conditions. One problem associated with the prior short circuit tack weld approach for pipe seams is relatively low amount of metal deposited per unit of time (low deposition rate), wherein the deposition rate is limited by the low heat limitation in order to prevent burn through. In this regard, a large current flows to the workpiece immediately after the short-circuit condition begins, which could potentially blow through the pipe. As a result, the arc voltage and/or current must be kept relatively low to avoid driving the welding arc through the workpiece seam, whereby the deposition rate of the buried arc approach is limited. Thus, conventional MIG arc welding and other prior welding processes are too slow for the demands of pipe welding operations, particularly for the initial pipe seam tack weld, without risking arc drive-through. Consequently, there is a continuing need for improved pipe seam tack welding processes and welding systems to provide high deposition rates and lineal welding speeds with improved seam integrity without undue spatter, arc drive-through, or excessive workpiece heating, for tack welding of longitudinal pipe seams in a pipe mill.